1. Field of the invention
The present invention concerns implementing an optical communication link when transmission of information over the link is subject to non-linear effects to the degree that correction of such effects is necessary. A link of this kind is typically a long-haul link such as an intercontinental link which can be from around 3 000 to 10 000 km long or even longer.
However, the present invention may be relevant to shorter links if they have a high mean optical power rating. To be more precise the invention applies when the integral of the power over the length of the line: .intg.P.dl, has high values producing non-linear effects capable of producing unacceptable transmission errors.
2. Description of the Prior Art
A known link of this kind comprises elements which are common to it and to a link in accordance with the present invention: these firstly comprise a transmission line in the form of an optical fiber having a normal direction chromatic dispersion less than 1 and preferably less than 0.5 ps/nm.km so that the dispersion of the line as a whole distorts a transmitted signal. This line is provided with optical amplifiers distributed over its length so that an optical signal guided by the fiber has locally increased amplitudes producing non-linear effects which further distort the signal.
These common elements secondly comprise means for correcting the optical signal which receive the line output signal and correct it to obtain a corrected optical signal closer to the line input signal. These signal correction means comprise an output filter adapted to filter optical frequencies. The bandwidth of this filter constitutes a corrective filtering bandwidth.
These common elements finally comprise optical signal processing means for receiving the corrected signal and for extracting therefrom the information to be transmitted which was conveyed by the input signal. The extracted information is inevitably affected by errors. The corrective filtering bandwidth is chosen to minimize the error rate.
In a first known long-haul link the corrective filtering bandwidth is very narrow. The aim is to eliminate as much as possible of the spectral components due to the non-linear effects. This bandwidth is 20 GHz, for example, that is to say 0.16 nm.
This first known long-haul link is described in the Journal of lightwave technology, vol 9, No 3, March 1991, (D. Marcuse).
A second known long-haul link is described in an article by N. Henmi et al "Dispersion compensation by prechirp technique in multigigabit optical amplifier repeater systems" "Topical Meeting on Optical Amplifiers and Their Applications", August 1990).
A particular object of the present invention is to enable simple implementation of a link of this kind having a reduced error rate and/or an increased bit rate.